A high electron mobility transistor, abbreviated HEMT, comprises a donor layer formed by a doped semiconductor layer, which may comprise n-doped GaAs or a compound of Ga, As and one or more further components like Al or In, particularly n-doped AlxGa1-xAs, for example. The donor layer is arranged adjacent to undoped semiconductor material having a narrower energy band gap, which can be undoped GaAs, for example, or a compound of Ga, As and one or more further components. Because of the heterojunction between the semiconductor materials of different energy band gaps, a quantum well is formed in the undoped semiconductor material in the vicinity of the donor layer. Electrons supplied by the donor layer diffuse into the quantum well. The electrons cannot escape from the quantum well and form a two-dimensional electron gas. The lack of impurities causes a low resistance and a high electron mobility. The electrons are thus confined to a high-mobility layer, which can be used as a channel region of a field-effect transistor. A gate electrode is applied to the surface of the donor layer. Dual-gate HEMTs or multiple-gate HEMTs comprise separate contacts forming the gate electrode similar to dual-gate or multiple-gate MOSFETs.
A pseudomorphic HEMT or pHEMT combines semiconductor materials of different lattice constants. One of the layers, which may be the layer comprising the high-mobility layer, is made sufficiently thin to adapt its crystal lattice to the lattice of the adjacent layer. This allows a larger difference of the energy band gaps, because the materials are not restricted to their own bulk lattice constant.
WO 2007/128075 A2 describes a lateral pnp transistor on GaAs. Ohmic contacts between the metal of the emitter and collector contacts and the semiconductor layers are achieved by a high doping concentration of the semiconductor material.
WO 2005/081768 A2 describes Schottky barrier junctions used in HBT structures.